1. Field of the Invention
The present invention relates to a power supply unit, which is constituted by directly connecting a fuel cell with an electric double layer capacitor
1. Description of the Related Art
Conventionally, a hybrid-type power supply unit, constituted by combining an electrochemical fuel cell (hereinafter, called a “fuel cell”) and an electric double layer capacitor (hereinafter, called a “capacitor”), has been known as a hybrid power source for driving an electric vehicle.
The hybrid-type power supply unit is capable of supplying power for a traction motor of an electric vehicle in a stable manner by compensating for the power deficiency of the fuel cell caused by the slow response of the fuel cell at the time of transitional variation of the load with energy stored in the capacitor.
As described above, since the response of the fuel cell for the transitional variation of the load is slow, the fuel cell, even combined with a capacitor, is unable to supply sufficient energy for the sudden variation of the load, which results in entering into, what is called, the “gas-shortage state”.
Accordingly, as a conventional measure to prevent a power deficiency, a DC/DC converter, provided between a fuel cell and a capacitor, is provided in order to control the output of the fuel cell. The DC/DC converter is used for controlling the output of the fuel cell so as not to exceed the output corresponding to an amount of reacting gas (including air and fuel gas) supplied to the fuel cell.
However, the DC/DC converter as an output control device controls the output of the fuel cell by a high speed switching (ON/OFF operation) of the switching element provided in the control device, and a problem arises in that the power loss due to the high speed switching operation becomes inevitably high.
In addition, in order to cope with a high output current of the fuel cell, the above output control device is designed to have a high current capacity, which causes a problem in that the control device must be large occupying a large amount of space in an electric vehicle.